The present invention relates to an information recording medium having information recorded thereon by a self-clocking modulation system which necessitates an area for producing an information reproducing clock, and a reproducing method therefor.
Many optical recording and reproducing apparatus which use recording media such as optical files or compact disks have been recently proposed, and cardlike optical recording media (optical card) which are more convenient to carry and have higher memory capacities per area than those recording media. Methods for utilizing such optical cards have also been proposed.
FIG. 1 is a plan view showing a record format of a prior art optical card, and FIG. 2 is a partial enlarged view thereof.
In FIGS. 1 and 2, a record area 2 is provided on an optical card 1 which is the recording medium. The record area 2 has a plurality of bands 3. Each of the bands 3 has a plurality of tracks 4 and each of the tracks 4 has several tens to one hundred bits of information capacity. The bands are separated by a reference line (R line) 5. An arrow A shows the direction of movement of the optical card 1 in the reproduction mode.
FIG. 3 shows a schematic configuration of an optical card reproducing apparatus.
In FIG. 3, the optical card 1 is movable in the direction of an arrow A by a rotation mechanism 6. The information recorded on the optical card 1 is read by an optical head 11 and reproduced track by track. A light from a light source 7 such as an LED is condensed by a lens system 8 to illuminate the track 4 on which the information is recorded, an image of the illuminated track 4 is focused onto a linear sensor array 10 by a focusing optical system 9 and the sensor array 10 produces an electrical signal representing the information recorded on the track 4. After the track 4 has been read, the optical card 1 is moved in the direction of the arrow A or the optical head 11 is moved across the bands 3 (in a direction of an arrow C) and the information on the next track is read.
Where the information on the track 4 is recorded by the NRZ (non-return to zero) system in which information "1" represents the presence of bit and information "0" represents the absence of bit, it is impossible to self-clock, that is, take out a clock signal (or demodulate without the clock signal) in the reproduction mode. Thus, the information must be reproduced by using a fixed clock such as a drive clock for the sensor array 10.
As a result, the size of the image of the track 4 focused onto the sensor array 10 is very important. Accordingly, it is necessary to precisely adjust the positional relationship of the optical card 1, focusing optical system 9 and sensor array 10 so that the desired image is focused on the sensor array 10. A high precision focusing optical system 9 is also required. As a result, the cost of the apparatus increases.
The above problems may be resolved if a self-clocking modulation system such as MFM, EFM, 4/5 NRZI or FM is used. However, when the information is recorded by such a self-clocking modulation system, the following problems are encountered.
In the modulation system other than the FM system, it is necessary to provide a preamble area in each track in order to take out an information reproducing clock. In order to take out a stable information reproducing clock, a preamble area of 10 to 20 bits is usually required. If preamble area of several tens of bits is provided in each track of the optical card, the data accommodation capacity is significantly reduced.
On the other hand, if an FM system is used, self-clocking is attained without the preamble area but the recording density is significantly reduced.